One aim of nuclear forensics is to establish the origin of nuclear and other radioactive material out of regulatory control. For uranium, characteristics such as age, i.e. the time passed since last chemical separation, isotopic composition, chemical phases and morphology, can be used to discriminate between different materials and determine e.g. the production process and thus identify the origin of the material. To determine the isotopic composition of uranium, the ratios n(234U)/n(238U), n(235U)/n(238U) and n(236U)/n(238U) must be determined. The content of 236U depends on contributions from anthropogenic sources and therefore the ratio n(236U)/n(238U) is an important nuclear signature. If the material contains relatively low amounts of 236U, sensitive measurement techniques must be used, for example TIMS or ICP-MS. Regardless of the technique, 236U is interfered with by the more abundant uranium isotopes 235U and 238U and these interferences must be quantified to allow for accurate determination of the n(236U)/n(238U) ratio. In this work the measurement technique ICP-SFMS has been used. Four different instrument set-ups have been studied, regarding spectral interferences from uranium, and correction factors for these interferences have been determined. Four certified reference materials were measured for the determination of the ratios n(234U)/n(238U), n(235U)/n(238U) and n(236U)/n(238U) using all instrument set-ups. The report shows that accurate results were obtained for n(234U)/n(238U), n(235U)/n(238U) and n(236U)/n(238U) for three of the reference materials, while only n(234U)/n(238U) and n(235U)/n(238U) could be determined accurately in the reference material in which the n(236U)/n(238U) ratio is very low. The conclusion from the work is that ICP-SFMS can be used for the determination of the isotope ratios n(234U)/n(238U), n(235U)/n(238U) and n(236U)/n(238U) in irradiated materials, but most likely not for samples in which there is no, or little, contribution from anthropogenic sources.